Patent application title: Metallic structure and photodetector

Abstract:

In a metallic structure including a metallic nano-chain with plasmon
resonance absorption, a metallic nanoparticle forming the metallic
nano-chain is formed in a circular, triangle, or rhomboid shape. The
wavelength selectivity can be increased also by forming a closed region
by mutually linking all of metallic nanoparticles that are mutually
linked with bottlenecks. In a photodetector, a photodetection unit
including a current detection probe, a nano-chain unit, and a current
detection probe is arranged on a substrate. The nano-chain unit is a
metallic structure with plasmon resonance absorption, where metallic
nanoparticles are mutually linked with bottlenecks. Each current
detection probe has a corner whose tip is formed with a predetermined
angle, and this corner is arranged to face the tip of the nano-chain
unit, i.e., a corner of the metallic nanoparticle. Photodetection with
high wavelength selectivity is performed based on a change in the initial
voltage of the current-voltage characteristic.

Claims:

1. A metallic structure comprising a metallic nano-chain with plasmon
resonance absorption, whereinthe metallic nano-chain is formed of a
plurality of metallic nanoparticles mutually linked with a plurality of
bottlenecks; andeach of the metallic nanoparticles is formed in any one
of a circular shape, a triangle shape, and a rhomboid shape.

2. The metallic structure according to claim 1, wherein the plurality of
bottlenecks linking the metallic nanoparticles are arranged on a straight
line.

3. The metallic structure according to claim 1, wherein the plurality of
bottlenecks linking the metallic nanoparticles are arranged on a
polygonal line.

4. The metallic structure according to claim 3, wherein the lengths of the
respective straight line portions in the polygonal line are formed so as
to differ from each other.

5. The metallic structure according to claim 1, wherein the metallic
nano-chain is arranged on a substrate.

6. A metallic structure comprising a metallic nano-chain with plasmon
resonance absorption, whereinthe metallic nano-chain is formed of a
plurality of metallic nanoparticles mutually linked with a plurality of
bottlenecks; andall of the plurality of metallic nanoparticles are
mutually linked to form a closed region.

7. The metallic structure according to claim 6, wherein the closed region
is circular.

8. The metallic structure according to claim 6, wherein the metallic
nano-chain is arranged on a substrate.

9. A photodetector comprising:a nano-chain unit having a plurality of
metallic nanoparticles mutually linked with a plurality of bottlenecks;
anda photodetection unit having a positive and a negative current
detection probes arranged on a substrate, whereinthe nano-chain unit with
plasmon resonance absorption is sandwiched between the positive and the
negative current detection probes.

10. The photodetector according to claim 9, whereinboth ends in a length
direction of the nano-chain unit are sandwiched between the positive and
the negative current detection probes, anda metallic nanoparticle at one
end of the nano-chain unit and a tip of at least one of the current
detection probes are arranged with a predetermined gap therebetween.

11. The photodetector according to claim 9, wherein a tip of a metallic
nanoparticle at one end of the nano-chain unit is arranged so as to be
sandwiched between the positive and the negative current detection probes
with a predetermined gap therebetween.

12. The photodetector according to claim 9, whereinboth sides of either
one of the metallic nanoparticles of the nano-chain unit are sandwiched
between the positive and the negative current detection probes, andat
least one of the current detection probes and one side surface of the
metallic nanoparticle are arranged with a predetermined gap therebetween.

13. The photodetector according to claim 9, wherein a predetermined
voltage is applied between the positive and the negative current
detection probes.

14. The photodetector according to claim 9, further comprising a plurality
of photodetection units formed to have different absorption wavelengths
of plasma resonance absorption caused by their respective nano-chain
units.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS AND INCORPORATION BY REFERENCE

[0001]This application is based upon and claims the benefit of prior
Japanese Patent Application P2007-228159 filed on Sep. 3, 2007 and
P2007-228160 filed on Sep. 3, 2007; the entire contents of which are
incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates to a metallic structure with plasmon
resonance absorption and a photodetector.

[0004]2. Description of the Related Art

[0005]A fine metallic body (e.g., a metallic microparticle having a
nanometer size) may exhibit an optical response called "local (surface)
plasmon resonance absorption" in a specific wavelength region among broad
wavelength regions ranging from the visible to infrared regions,
depending on the shape or size of the metallic body. The examples of the
metal exhibiting the local plasmon resonance absorption include noble
metals, such as gold, silver, and platinum; however, even if the metal
type is the same, if the size or shape of the metal differs, the local
plasmon resonance absorption wavelength also differs. Attempts have been
made to apply, to various optical devices, the nature that the absorption
wavelength varies depending on a difference in size or shape of fine
metallic bodies.

[0006]A metallic structure having a plurality of fine metallic bodies
arranged in a substrate may have the plasmon resonance absorption in a
broad region ranging from the visible to the infrared region, based on
the principle of the local (surface) plasmon. In attempting to apply such
metallic structure to an optical device or a sensor, it is important to
adjust the wavelength region of this plasmon resonance absorption.

[0007]Moreover, for example, as shown in "Applied Physics A, vol. 29, pp.
71-75 (1982)," a phenomenon in which infrared absorption is enhanced by
an optical electric field enhancement phenomenon via the plasmon at the
surface of a metallic structure has been found. However, the mechanism
thereof has not been clarified yet, and a quantitative measurement method
or the like of the enhanced absorption has not been established yet,
either. Therefore, if a metallic structure having a plasmon resonance
frequency in a desired infrared region can be fabricated, an optical
device or measurement system using the above-described phenomenon can be
constructed.

[0008]The wavelength region of the plasmon resonance absorption which the
above-described metallic structure has is affected by the "slenderness
(the aspect ratio if the fine metallic body is rod-shaped)" of the fine
metallic body. In other words, if a slender (the aspect ratio is high)
fine metal is used, the wavelength region of the plasmon resonance
absorption shifts to the long wavelength side, while if a short (the
aspect ratio is low) fine metal is used, the wavelength region of the
plasmon resonance absorption shifts to the short wavelength side.

[0009]On the other hand, for example, as shown in "J. Phys. Chem. B, 108,
13066(2004)" and "JACS, 125, 13915 (2003)," techniques for chemically
bonding and linking a plurality of rod-shaped nano metallic (gold: Au)
bodies are known. The nano metallic bodies linked by a chemical bond
(e.g., streptavidin-biotin interaction) are coupled via a chemical
substance, which is not a metal, and strictly speaking, the nano metallic
bodies are not directly linked to each other.

[0010]As described above, in order to obtain a metallic structure having
the plasmon resonance absorption on the long wavelength side (e.g.,
infrared region), it is necessary to form a slender fine metallic body
(e.g., a rod-shaped fine metallic body having a high aspect ratio).
However, a single slender fine metallic body would cause a multi-mode
absorption to deteriorate the wavelength selectivity, causing a problem
in use in an optical device or the like. Then, it is contemplated that a
metallic structure linking the nano metallic bodies is formed to improve
the wavelength selectivity.

[0011]However, even with the metallic structure made by linking the nano
metallic bodies, the absorption peak width of plasmon resonance
absorption is not become very sharp. In order to increase the wavelength
resolution and improve the wavelength selectivity further, the absorption
peak width is expected to be sharpened further.

[0012]On the other hand, a photodetector has not been achieved yet that
uses a metallic structure having the resonance wavelength of local
plasmon on the long wavelength side while not exhibiting a multi-mode
absorption (i.e., wavelength selectivity is high), and that achieve the
detection of light in the terahertz region.

SUMMARY OF THE INVENTION

[0013]The present invention has been made to solve the above-described
problems. It is an object of the present invention to provide a metallic
structure and a photodetector having a plasmon resonance absorption
effect and improved wavelength selectivity.

[0014]In order to achieve the above-described object, the invention
according to Claim 1 is a metallic structure including a metallic
nano-chain with plasmon resonance absorption, in which the metallic
nano-chain is formed a plurality of metallic nanoparticles mutually
linked with a plurality of bottlenecks, and each of the metallic
nanoparticles is formed in any one of a circular shape, a triangle shape,
and a rhomboid shape.

[0015]Moreover, the invention according to Claim 2 is the metallic
structure according to Claim 1, in which the plurality of bottlenecks
linking the metallic nanoparticles are arranged on a straight line.

[0016]Moreover, the invention according to Claim 3 is the metallic
structure according to Claim 1, in which a plurality of bottlenecks
linking the metallic nanoparticles are arranged on a polygonal line.

[0017]Moreover, the invention according to Claim 4 is the metallic
structure according to Claim 3, in which the lengths of the respective
straight line portions in the polygonal line are formed so as to differ
from each other.

[0018]Moreover, the invention according to Claim 5 is the metallic
structure according to Claim 1, in which the metallic nano-chain is
arranged on a substrate.

[0019]Moreover, the invention according to Claim 6 is a metallic structure
including a metallic nano-chain with plasmon resonance absorption, in
which the metallic nano-chain is formed of a plurality of metallic
nanoparticles mutually linked with a plurality of bottlenecks, and all of
the plurality of metallic nanoparticles are linked to form a closed
region.

[0020]Moreover, the invention according to Claim 7 is the metallic
structure according to Claim 6, wherein the closed region is circular.

[0021]Moreover, the invention according to Claim 8 is the metallic
structure according to Claim 6, wherein the metallic nano-chain is
arranged on a substrate.

[0022]Moreover, the invention according to Claim 9 is a photodetector
including: a nano-chain unit having a plurality of metallic nanoparticles
mutually linked with a plurality of bottlenecks; and a photodetection
unit having a positive and a negative current detection probes arranged
on a substrate, in which the nano-chain unit with plasmon resonance
absorption is sandwiched between the positive and the negative current
detection probes.

[0023]Moreover, the invention according to Claim 10 is the photodetector
according to Claim 9, in which both ends in the length direction of the
nano-chain unit are sandwiched between the positive and the negative
current detection probes, and a metallic nanoparticle at one end of the
nano-chain unit and a tip of at least one of the current detection probes
are arranged with a predetermined gap therebetween.

[0024]Moreover, the invention according to Claim 11 is the photodetector
according to Claim 9, in which a tip of a metallic nanoparticle at one
end of the nano-chain unit is arranged so as to be sandwiched between the
positive and the negative current detection probes with a predetermined
gap therebetween.

[0025]Moreover, the invention according to Claim 12 is the photodetector
according to Claim 9, in which both sides of either one of the metallic
nanoparticles in the nano-chain unit are sandwiched between the positive
and the negative current detection probes, and wherein at least one of
the current detection probes and one side surface of the metallic
nanoparticle are arranged with a predetermined gap therebetween.

[0026]Moreover, the invention according to Claim 13 is the photodetector
according to Claim 9, in which a predetermined voltage is applied between
the positive and the negative current detection processes.

[0027]Moreover, the invention according to Claim 14 is the photodetector
according to Claim 9, further including a plurality of photodetection
units formed to have different absorption wavelengths of plasma resonance
absorption caused by their respective nano-chain units.

[0028]According to the metallic structure of the present invention, since
the shape of each of the metallic nanoparticles mutually linked with
bottlenecks is formed in any one of a circular shape, a triangle shape,
or a rhomboid shape, the shape of a plasmon resonance absorption peak
spectrum can be sharpened and the wavelength resolution can be improved.
On the other hand, also by forming a closed region by mutually linking
all of the plurality of metallic nanoparticles that are mutually linked
with bottlenecks, the shape of the plasmon resonance absorption peak
spectrum can be sharpened and the wavelength resolution can be increased.

[0029]On the other hand, according to the photodetector of the present
invention, since the wavelength selectivity is high, the light in the
terahertz region can be detected precisely and the intensity of the light
can be also detected.

BRIEF DESCRIPTION OF DRAWINGS

[0030]FIG. 1A to FIG. 1C are schematic views showing structural examples
of a metallic nano-chain in a metallic structure of the present
invention.

[0031]FIG. 2 is a schematic view showing the length of a bottleneck of
each of metallic nano-chains.

[0032]FIG. 3A is a schematic view showing the length of a metallic
nano-chain, and FIG. 3B is a schematic view showing the neck width of a
bottleneck.

[0033]FIG. 4 is a schematic view showing a structural example of a
metallic nano-chain.

[0034]FIG. 5 is a view showing a relationship between the number of
metallic nanoparticles and the peak wavelength when each of the metallic
nano-chains of FIG. 2 is used.

[0035]FIG. 6 is a view showing a relationship between the peak frequency
corresponding to the data of FIG. 5 and the FWHM of a spectrum.

[0036]FIG. 7A to FIG. 7B are schematic views showing a bending structure
of a metallic nano-chain.

[0037]FIG. 8A to FIG. 8c are schematic views showing examples of the
bending structure of a metallic nano-chain.

[0038]FIG. 9 is a view showing a relationship between a resonance
absorption frequency and the absorbancy of a metallic nano-chain having a
bending structure.

[0039]FIG. 10 is a schematic view showing a structural example when a
metallic nano-chain has a closed region.

[0040]FIG. 11 is a view showing, in the case where a metallic nano-chain
has a circular linked structure, differences in the absorbancy when the
link is a closed region and when the link has a gap.

[0041]FIG. 12 is a view showing data when the metallic nano-chain has a
circular closed region is added to the data of FIG. 6.

[0042]FIG. 13 is a view showing the shot data when the shape of a metallic
nano-chain does not form a closed region but has a gap.

[0043]FIG. 14 is a view showing the polarization dependency of a metallic
nano-chain.

[0044]FIG. 15 is a view showing the polarization dependency of a metallic
nano-chain.

[0045]FIG. 16 is a view showing an example of the steps of manufacturing a
metallic nano-chain.

[0046]FIG. 17 is a view showing a relationship between the number of
metallic nanoparticles and the length of a metallic nano-chain.

[0047]FIG. 18A and FIG. 18B are views showing the local plasmon bands of
metallic nano-chains.

[0048]FIG. 19A and FIG. 19B are views showing a structure of a
photodetector of the present invention.

[0049]FIG. 20 is a schematic view showing current-voltage characteristic
when the photodetector shown in FIG. 19A and FIG. 19B is irradiated with
light and plasmon resonance absorption occurs.

[0050]FIG. 21 is a view showing I-V characteristics when a gap between the
current detection probes and a nano-chain unit is irradiated with a laser
beam.

[0051]FIG. 22 is a view showing a change when converting the data of FIG.
21 to a relationship between the LD driving current and the initial
voltage.

[0052]FIG. 23 is a view showing the I-V characteristics when being
irradiated with the light of a light source lamp and when not being
irradiated, respectively.

[0053]FIG. 24A to FIG. 24c are views showing examples of the arrangement
of current detection probes on the positive electrode and negative
electrode and a nano-chain unit.

DETAILED DESCRIPTION OF THE INVENTION

[0054]Hereinafter, one embodiment of the present invention will be
described with reference to the accompanying drawings. FIG. 1A to FIG. 1C
show plan views of metallic nano-chain units having a metallic structure
of the present invention.

[0055]A metallic nano-chain is a metallic body with plasmon resonance
absorption, wherein a plurality of metallic nanoparticles are mutually
linked with bottlenecks. In FIG. 1A the plane shape of each metallic
nanoparticle 1 is formed in a circular shape, in FIG. 1B the plane shape
of each metallic nanoparticle 2 is formed in an isosceles triangular
shape, and in FIG. 1C the plane shape of each metallic nanoparticle 3 is
formed in a rhomboid shape. Here, the bottleneck indicates a portion
where parts of the metallic nanoparticles are overlappingly formed as
shown in FIG. 2 and FIG. 3B. Accordingly, one nanoparticle is slightly
overlapped with an adjacent nanoparticle, thereby allowing a free
electron contained in the one nanoparticle to move to the adjacent
nanoparticle to a certain extent.

[0056]Moreover, the material of the metallic nanoparticle may be of any
metal that provides surface plasmon absorption by being a nanoparticle,
and the examples thereof include noble metals, such as gold, silver, and
platinum. Moreover, the metallic nanoparticle may be a nano substance
made from other material coated with such metal.

[0057]As described above, in the metallic nano-chain, a plurality of
metallic nanoparticles are mutually linked with bottlenecks. When there
are a plurality of bottlenecks in this manner (i.e., no less than three
metallic nanoparticles are linked), the center of each of the bottlenecks
is arranged on a straight line as indicated by a dotted line shown in
FIG. 1A to FIG. 1C. As the arrangement method, the metallic nano-chain
may be arranged so as to be horizontally symmetrical about the
arrangement line of the bottleneck center as shown in FIG. 1 A to FIG. 1C
(vertically symmetrical in the view). However, instead of being
horizontally symmetrical, if the shape of the metallic nanoparticle is
triangle or the like, the base of each triangle may be linked on a
straight line so as to arrange the triangle only on one side using the
base as a border. In FIG. 1A to FIG. 1C, eight metallic nanoparticles are
linked, however, since the bottleneck center is arranged on a straight
line, a free electron can easily move between the microparticles through
the respective bottlenecks.

[0058]On the other hand, FIG. 4 shows a case where the plane shape of each
of the metallic nanoparticles forming a metallic nano-chain is formed in
a square shape. A plurality of metallic nanoparticles 5 are mutually
linked with bottlenecks. If this link has a plurality of bottlenecks
(i.e., no less than three metallic nanoparticles are linked), the
respective bottleneck centers are preferably arranged on a straight line
as shown in FIG. 4. In FIG. 4, four metallic nanoparticles are linked,
but the bottleneck centers arranged on a straight line would facilitate
the movement of a free electron between the microparticles through the
respective bottlenecks.

[0059]Since the metallic nanoparticle to be linked has a thickness, the
shape in FIG. 1A is cylindrical, the shape in FIG. 1B is triangular prism
shaped, the shape in FIG. 1C and FIG. 4 is square pole-shaped in terms of
three-dimensional shape, respectively. When the shape of a metallic
nanoparticle is triangular prism shaped or square pole shaped, the
bottleneck is preferably formed by the metallic nanoparticles being
linked to each other at their edge lines. This allows the neck width of
the bottleneck to be reduced easily.

[0060]In forming a metallic nanoparticle on a substrate, the plane of the
metallic nanoparticle is preferably arranged so as to be equal in level
to the substrate surface. The volume of deposition of the metallic
nanoparticle to be linked is preferably in the order of 100,000 nm3
to 1,000,000 nm3. Furthermore, the area of the metallic nanoparticle
in a plan view is preferably in the order of 5,000 nm2 to 20,000
nm2. Moreover, the height of the metallic nanoparticle from the
substrate is preferably in the order of 10 to 100 nm.

[0061]The number of metallic nanoparticles to be linked is preferably in
the order of 2 to 50. Since the absorption resonance wavelength is
proportional to the number of metallic nanoparticles, the number of
metallic nanoparticles to be linked (the length of a metallic body
extending through the bottlenecks) may be suitably selected in accordance
with a desired resonance absorption wavelength.

[0062]In the metallic structure of the present invention, the metallic
nanoparticle forming a metallic nano-chain is formed in either a circular
shape, a triangle shape, or a rhomboid shape as shown in FIG. 1A to FIG.
1C, thereby sharpening the peak width of plasmon resonance absorption and
improving the wavelength resolution.

[0063]Hereinafter, the experiment results of the wavelength resolution are
described. FIG. 2 showed the bottleneck portion of the metallic
nanoparticles having a different shape, wherein in FIG. 2(a) the plane
shape of the metallic nanoparticle is a square shape, in FIG. 2(b) the
plane shape of the metallic nanoparticle is a circular shape as in FIG.
1A, in FIG. 2(c) the plane shape of the metallic nanoparticle is an
isosceles triangular shape as in FIG. 1B, and in FIG. 2(d) the plane
shape of the metallic nanoparticle is a rhomboid shape as in FIG. 1C.

[0064]FIGS. 2(a), 2(b), 2(c), and 2(d) show the neck lengths t1, t2, t3,
and t4 of the bottlenecks that link the metallic nanoparticles, where
t1=t2=t3=t4=4.5 nm. The smaller the neck length, the further the
multi-mode absorption of the metallic nano-chain is removed and the
further the wavelength selectivity is improved. On the other hand, if the
neck length is too small, the scattering of a plasmon electron in the
vicinity of the bottleneck may become large. Moreover, also the number of
links of the metallic nanoparticles of the metallic nano-chain is made
the same, and the metallic nano-chain was configured so that the length X
thereof at this time may be equal for each of FIGS. 2(a), 2(b), 2(c), and
2(d). When for example two metallic nanoparticles are linked as shown in
FIG. 2, the length X is formed so as to be set to 282 nm, and the
thickness of the metallic nanoparticle having each shape is set to 30 nm.

[0065]In order to actually fabricate the metallic nano-chain, a substrate
serving as the base is required, and any substrate capable of arranging
therein the metallic nanoparticles may be employed. Furthermore, the
substrate is preferably a solid substrate, wherein at least the surface
in which the metallic nanoparticles are arranged is made from an
insulator. Moreover, the substrate is preferably a substrate made from a
material that does not absorb light (e.g., light in the visible region to
the near infrared region) incident from the outside, for example
comprising a transparent substrate. Since the metallic nano-chain is
manufactured using semiconductor fine processing techniques (e.g.,
electron beam lithography, sputtering, and the like) as described later,
the substrate needs to withstand these processings. Accordingly, for
example, a glass substrate, a quartz substrate, a sapphire substrate, and
the like are used as the substrate.

[0066]Next, a manufacturing method in forming the metallic nano-chain on a
substrate is described hereinafter with reference to FIG. 16. The
metallic nano-chain is preferably manufactured using the semiconductor
fine processing techniques. For example, as shown in FIG. 16(a), a
substrate 4 is prepared, and the surface of the solid substrate 4 is
coated with a resist 11 as shown in FIG. 16(b). Then a desired shape of
the nano metallic body is drawn onto the resist 11 using an electron
beam. Next, the drawn pattern is developed to expose the substrate in
accordance with the shape of the nano metallic body, as shown in FIG.
16(c). Thereafter, as shown in FIG. 16(d), a metal is sputtered onto the
developed surface to form a metal film 12, and then an unwanted metal
film is removed along with the resist using lift-off to form a metallic
nano-chain formed by linking the metallic nanoparticles, as shown in FIG.
16(e).

[0067]The method of manufacturing a metallic nano-chain can be carried out
similarly to the method of manufacturing a metallic structure described
in Japanese Patent Application No. 2005-080579 or Japanese Patent
Application No. 2005-258364, for example. One of the important
manufacturing conditions is the film thickness of the resist to be coated
onto the substrate. This film thickness is preferably set to 200 nm or
less. Moreover, it is preferable to reduce the concentration of a coating
resist solution in order to reduce the film thickness.

[0068]Another one of the important manufacturing conditions is the
exposing condition of the electron beam in the step of drawing a desired
shape of a nano metallic body onto the resist using the electron beam.
That is, it is preferable to increase the acceleration voltage of the
electron beam and at the same time to reduce the exposure dose rate. More
specifically, it is preferable to set the acceleration voltage of the
electron beam to 100 kV to 200 kV and set the exposure dose rate to 2
μC/cm2 or less.

[0069]Another one of the important manufacturing conditions is the
development condition to remove the drawn resist, i.e., the development
time in particular. Since the exposure dose rate is small, it is
preferable to lengthen the development time, e.g., to carry out the
development for about 30 minutes.

[0070]The plurality of bottleneck centers in the linked metallic
nanoparticles are preferably arranged on a straight line as described
above, however, as described later, the length of the overall metallic
nano-chain on this straight line might be in the order of 0.2 μpm to 4
μm, preferably in the order of 0.2 μm to 2.0 μm. The length of
the metallic nano-chain is adjusted depending on the size of the metallic
nanoparticles to be linked, the number of the metallic nanoparticles to
be linked, or the like. If the length of the metallic nano-chain is
lengthened, the plasmon resonance absorption wavelength of the metallic
structure will shift to the long wavelength side.

[0071]Although the above contents is described in detail in Japanese
Patent Application No. 2006-182637, the dependence of the plasmon
resonance absorption wavelength particularly on the length X of the
metallic nano-chain is shown below.

[0072]First, a metallic nano-chain was formed on a sapphire substrate as
follows. The surface of the sapphire substrate (10 mm×10 mm) is
ultrasonically cleaned for three minutes each using acetone, methanol,
and ultra-pure water in this order. The positive type electron-beam
resist (Zep-520a; made by Nippon Zeon Co., Ltd.) was spin-coated (at
4,000 rpm) onto the cleaned substrate surface to form a resist film (in
the thickness of 200 nm). A desired pattern of a metallic nano-chain 2
was drawn at the dose rate of 1.2 μC/cm2 using an electron beam
exposure system with the acceleration voltage of 100 kV. The development
was carried out for 30 minutes, and then the resultant substrate was
rinsed and dried.

[0073]Next, gold (Au) was sputtered onto the substrate to form a metal
film (40 nm). The substrate having the metal film formed thereon was
immersed in a resist remover solution, and ultrasonic cleaning was
carried out to remove the resist, and the remaining resist was lifted
off.

[0074]Metal bodies having a shape linking 1 to 25 rectangular
parallelepiped-shaped metallic nanoparticles were formed on the sapphire
substrate, respectively, to obtain a metallic structure having a
plurality of metallic nano-chains formed thereon. The composition of the
metallic nanoparticle is gold (Au). Each metallic nanoparticle of each
nano-block shaped metallic nano-chain was formed in a rectangular
parallelepiped shape which has a square shape of 100 nm×100 nm when
viewed from the upper surface of the substrate, and a height of 40 nm
from the substrate. The bottleneck to be linked by the edge lines of the
rectangular parallelepiped was formed to set the neck width of this
bottleneck to 4.4 nm as shown in FIG. 3B. That is, in FIG. 2(a), the neck
length in the direction perpendicular to t1 was set to 4.4 nm. The
directions of the metallic nano-chains formed on the substrate were made
the same, respectively, and the interval between the metallic nano-chains
was set to 1,000 nm as a constant.

[0075]The respective metallic structures obtained through the
above-described formation were irradiated from the upper surface with
light having the wavelength of 660 nm to 7,142 nm (wave number of 15,000
cm-1 to 1,400 cm-1) to measure the absorbancy using a
microscopic FT-IR measuring apparatus. The obtained results are shown in
FIG. 18A and FIG. 18B. FIG. 18A shows the data of a metallic structure,
in which metallic nano-chains whose number of linked metallic
nanoparticles "n" is from 1 to 7 are arranged, while FIG. 18B shows the
data of a metallic structure, in which the metallic nano-chains whose
number of linked metallic nanoparticles "n" is from 6 to 25 are arranged.
For the spectrum data of the metallic structure in which the metallic
nano-chain whose number of metallic nanoparticles "n" is one is arranged,
the peak thereof is not shown here, but this is because the peak exists
outside the display area (on the higher energy side). The relationship
between the number of metallic nanoparticles "n" and the length X of the
metallic nano-chain is as shown in FIG. 17.

[0076]As shown in FIG. 18A and FIG. 18B, it can be seen that as the length
X of the metallic body is lengthened by increasing the number of linked
metallic nanoparticles "n", the half-value width of the spectrum
decreases and light (light having a long wavelength) in the region having
a smaller photon energy is absorbed. This may be because the resonance
wavelength has shifted to the long wavelength side, thereby lengthening
the phase relaxation time of plasmon.

[0077]As shown in FIG. 18A and FIG. 18B, for the metallic structure having
the metallic nano-chain of linked metallic nanoparticles arranged
therein, only dipole-mode plasmon resonance absorption was observed. This
may be because only a plasmon resonance band based on a free electron
traversing a nano-contact in the bottleneck is observed.

[0078]As described above, it was found that the wavelength of light to be
resonantly absorbed can be varied by varying the length X of the metallic
nano-chain.

[0079]Next, metallic nano-chains formed of the metallic nanoparticles
shown in FIG. 1A to FIG. 1C were fabricated on the sapphire substrate, as
described above. Moreover, for comparison, a metallic nano-chain formed
of the square-shaped metallic nanoparticles shown in FIG. 2(a) and FIG. 4
was also fabricated. In this case, as described above, the neck lengths
t1, t2, t3, and t4 of the bottlenecks shown in FIG. 2 were set as
t1=t2=t3=t4=4.5 nm. Moreover, also the number of links of the metallic
nanoparticles of the metallic nano-chain was made the same, and the
metallic nano-chain was configured so that the length X thereof at this
time may be equal for each of FIGS. 2(a), 2(b), 2(c), and 2(d). When two
metallic nanoparticles are linked to each other as shown in FIG. 2, the
metallic nano-chain is formed so that the length X is set to 282 nm, and
the thickness of the metallic nanoparticle having each shape is set to 30
nm.

[0080]The respective metallic structures obtained through the
above-described formation were irradiated from the upper surface with
light having the wavelength of 660 nm to 7,142 nm (wave number of 15,000
cm-1 to 1,400 cm-1) to measure the absorbancy, using a
microscopic FT-IR measuring apparatus. The obtained results are shown in
FIG. 5 and FIG. 6.

[0081]The vertical axis of FIG. 5 represents the peak wavelength of the
absorbed light, and the horizontal axis represents the number of metallic
nanoparticles. Moreover, a filled square represents the data from the
square-shaped metallic nanoparticle for comparison shown in FIG. 2(a), a
filled circle represents the data from the metallic nano-chain of FIG.
1A, a filled triangle represents the data from the metallic nano-chain of
FIG. 1B, and a filled rhomboid shape represents the data from the a
metallic nano-chain of FIG. 1C. As the number of metallic nanoparticles
increases and the length of the metallic nano-chain accordingly
increases, the peak energy will decrease as described above and therefore
the peak wavelength will increase.

[0082]FIG. 6 is a graph of the FWHM (half-value width) of the peak
frequency of the respective absorption spectra that were measured with
respect to the data measured in FIG. 5. The vertical axis represents the
FWHM (THz) and the horizontal axis represents the peak frequency f (THz).
The data plotted in FIG. 6 corresponds to the data of FIG. 5. Here, as
the number of metallic nanoparticles increases, the peak wavelength
increases while the peak frequency decreases. Therefore the direction of
the horizontal axis of FIG. 6 corresponds to a sequence from the data for
a large number of metallic nanoparticles to the data for a small number
of metallic nanoparticles.

[0083]It can be seen that in the case of the square-shaped metallic
nanoparticle (data indicated by the filled square in the view), the FWHM
is rather large across the detected peak frequencies as compared with the
case where the circular, triangular, or rhomboid-shaped metallic
nanoparticle is used. By forming the planar shape of the metallic
nanoparticle constituting the metallic nano-chain in either a circular
shape, a triangular shape, or a rhomboid shape in this way, narrower band
of wavelengths can be absorbed as compared with the case where the
square-shaped metallic nanoparticle is used. On the other hand, if the
square shape is selected, broad band of wavelengths can be absorbed. The
shape of the nanoparticle may be suitably selected in accordance with a
desired wavelength resolution.

[0084]Next, a structure having a plurality of plasmon resonance absorption
wavelengths (peak wavelengths), instead of one plasmon resonance
absorption wavelength, caused by the metallic nano-chain is described
below. FIG. 7A shows a metallic nano-chain structure having one resonance
absorption wavelength λ1, while FIG. 7B shows a metallic nano-chain
structure having a plurality of resonance absorption wavelengths
λ1, λ2, and λ3.

[0085]The structure of FIG. 7A is the same as that of FIG. 1A described
above, wherein the resonance absorption wavelength λ1 is determined
by the length of the metallic nano-chain, and there is only one resonance
wavelength. However, if the bottlenecks are arranged not on a straight
line but on a polygonal line as shown in FIG. 7B, the resonance
absorption wavelengths λ2, λ3 corresponding to the lengths of
the straight line portions in the polygonal line are additionally
generated. Here, λ1 >λ2 and λ1, >λ3. Then,
in such a case as in FIG. 7B, the absorption peak appears at three places
λ1, λ2, and λ3 including the absorption wavelength
λ1 that is based on the length of the overall polygonal line. In
this manner, even in a narrow region, a metallic nano-chain having
sensitivity at a plurality of wavelengths can be fabricated.

[0086]FIG. 9 shows that the bending angle θ shown in FIG. 7B
produces a change in the intensity of the resonance absorption peak. The
shape of the metallic nanoparticle was circle as in FIG. 7B, and the
number of metallic nanoparticles constituting the metallic nano-chain was
set to nine. Moreover, the bottlenecks of six metallic nanoparticles were
linearly linked, and, in order to have the bending angle θ with
respect to this straight line, the bottlenecks of other three metallic
nanoparticles were arranged on a straight line. The bending angle θ
was varied from 0 to 80 degrees by 10 degrees intervals. Moreover, three
numbers shown on each peak represent the numbers of the metallic
nanoparticles. In this way, the resonance absorption wavelength
corresponding to the length of each straight line portion of the
polygonal line (i.e., corresponding to the length of each metallic
nano-chain when each metallic nano-chain comprises three, six, or nine
metallic nanoparticles) will appear. Here, if particularly the peak of
the absorption wavelength λ1 based on the length of the overall
polygonal line is going to be strengthened, all the bottlenecks had
better be arranged on a straight line if possible, and the bending angle
θ is preferably set to 10 degrees or less.

[0087]Moreover, as in FIG. 7B, the metallic nano-chain having a bending
structure and having sensitivity at a plurality of wavelengths may
comprise a metallic nanoparticle whose planar shape is formed in a
triangular shape or a rhomboid shape as shown in FIG. 8B, FIG. 8c, other
than in a circular shape. In this case, as in the data of FIG. 9, the
metallic nano-chain can be formed so as to have sensitivity at a
plurality of wavelengths.

[0088]In FIG. 10, all the circular metallic nanoparticles are mutually
linked with bottlenecks to form a closed region in the metallic
nano-chain. In this embodiment, the closed region is formed in a circular
shape. Here, the radius from the center of the circular closed region to
the bottleneck center is denoted as R, and the angle between the center
of a metallic nanoparticle and the center of the adjacent metallic
nanoparticle is denoted as α. Note that the metallic nanoparticle
may be formed not only in a circular shape but also in a triangular
shape, a rhomboid shape, a square shape, or the like. Moreover, the
resonance absorption wavelength of the metallic nano-chain is determined
by the size of the closed region, i.e., the radius R.

[0089]FIG. 11 shows a relationship between the resonance absorption
wavelength (nm) and the absorbancy when the overlapping portion of the
bottleneck is gradually made larger (i.e., α is varied from 40
degrees to 39.5 degrees, 39 degrees, and 38 degrees) in the configuration
of FIG. 10 while maintaining the circular shape of the link. When the
angle α is large, the overlapping portion of the bottleneck is
small and the region surrounded by the metallic nanoparticles that are
mutually linked with the bottlenecks still maintains the closed region,
however, when a reaches 38 degrees, as shown in FIG. 13, a gap will occur
between some of the metallic nanoparticles and the closed region cannot
be formed anymore. Then, not only a peak occurs at a predetermined
absorption wavelength but also an absorption peak called a gap mode
occurs in other wavelength regions, which is seen in the absorption
spectrum when a shown in FIG. 11 is 38 degrees. Therefore, in the case
where the metallic nano-chain forms a closed region, the light collection
efficiency due to the resonance absorption is improved more than the case
where there is a gap in the metallic nano-chain. When the metallic
nano-chain forms a closed region, the light collection efficiency reaches
as high as 70%.

[0090]On the other hand, FIG. 12 is a view of the same graph as that of
FIG. 6 adding the data indicated by an open circle. The data indicated by
the open circle corresponds to the measurement of the FWHM of the
spectrum of the resonance absorption frequency of the circular closed
region that is formed by using the circular metallic nanoparticles as in
FIG. 10. As apparent from the view, the FWHM has been improved more and
the spectral width becomes smaller as compared with the data indicated by
the filled circle in the case where a plurality of circular metallic
nanoparticles are arranged on a straight line. Therefore, the wavelength
resolution has been improved significantly than at least that of the
metallic nano-chain having a plurality of square-shaped metallic
nanoparticles arranged on a straight line.

[0091]Moreover, the advantages when the metallic nano-chain forms a closed
region reside also in not having polarization dependency but having
isotropy. FIG. 14 and FIG. 15 show the isotropic data. FIG. 15 shows a
relationship between the absorption wavelength (horizontal axis) and the
absorbancy (vertical axis) when the structure of FIG. 10 or FIG. 13 is
irradiated with a vertically polarized wave. In this case, there is no
particular change between when the metallic nano-chain maintains the
closed region and when it has a gap. FIG. 14 shows a relationship between
the absorption wavelength (horizontal axis) and the absorbancy (vertical
axis) when the structure of FIG. 10 or FIG. 13 is irradiated with a
horizontally polarized wave. In this case, there is a significant shift
particularly in the peak wavelength when the metallic nano-chain has a
gap (corresponding to the curves of 37 degrees, 38 degrees, and 38.5
degrees).

[0092]Next, an embodiment of a photodetector of the present invention is
described. FIG. 19A shows a plan view (top view) of the photodetector of
the present invention, and FIG. 19B shows a cross sectional view of FIG.
19A.

[0093]A photodetection unit including a current detection probe 21, a
nano-chain unit 22, and a current detection probe 23 is arranged on a
substrate 24. The nano-chain unit 22 is a metallic structure with plasmon
resonance absorption, wherein a plurality of metallic nanoparticles 22a
are mutually linked with bottlenecks. Here, the bottleneck indicates a
portion where parts of the metallic nanoparticles 22a are overlappingly
formed as described in FIG. 2 or FIG. 3B. That is, one nanoparticle 22a
is slightly overlapped with an adjacent nanoparticle 22a, thereby
allowing a free electron contained in the one nanoparticle to move to the
adjacent nanoparticle to a certain extent.

[0094]Moreover, the material of the metallic nanoparticle 22a may be of
any metal that provides surface plasmon absorption by being a
nanoparticle, and the examples thereof include noble metals, such as
gold, silver, and platinum. Moreover, the metallic nanoparticle 22a may
be a nano-substance made from other material coated with such metal.

[0095]As described above, in the nano-chain unit 22, a plurality of
metallic nanoparticles 22a are mutually linked with bottlenecks.
Moreover, if there are a plurality of bottlenecks (i.e., no less than
three metallic nanoparticles are linked), the respective bottleneck
centers are preferably arranged on a straight line, as described in the
paragraph about the metallic structure. The bottleneck centers arranged
on a straight line would facilitate the movement of a free electron
between the microparticles through the respective bottlenecks.

[0096]The shape of the metallic nanoparticle 22a to be linked comprises,
for example, such shapes shown in FIGS. 2(a) to 2(d), and is rectangular
parallelepiped, for example, although not limited in particular. When the
shape of the metallic nanoparticle is rectangular parallelepiped, the
bottleneck is preferably formed by linking the metallic nanoparticles at
their edge lines. Thereby, the neck width of the bottleneck can be
reduced easily.

[0097]When viewed from vertically above with respect to the substrate,
that is in a plan view, as in FIGS. 2(a), 2(c), and 2(d), it is
preferable that the metallic nanoparticle be polygonal and have a corner.
Furthermore, the shape of the metallic nanoparticle to be linked is
preferably the rectangular parallelepiped having the surfaces with a
square shape and a rectangular shape as shown in FIGS. 2(a), 2(d), and
the surface of the square shape is preferably arranged so as to be equal
in level to the substrate surface. In other words, as in FIG. 2(a), it is
preferable that the metallic nanoparticle looks square in a plan view.

[0098]The volume of deposition of the metallic nanoparticle to be linked
is preferably in the order of 100,000 nm3 to 1,000,000 nm3.
Furthermore, the area of the metallic nanoparticle when viewed from the
upper surface of the substrate is preferably in the order of 5,000
nm2 to 20,000 nm2. Moreover, the height of the metallic
nanoparticle from the substrate is preferably in the order of 10 to 100
nm.

[0099]The number of metallic nanoparticles to be linked is preferably in
the order of 2 to 50. Since the absorption resonance wavelength is
proportional to the number of metallic nanoparticles, the number of
metallic nanoparticles to be linked (the length of a metallic body
extending through the bottlenecks) may be suitably selected in accordance
with a desired resonance absorption wavelength.

[0100]Next, although the substrate 24 may be a substrate capable of
arranging the metallic nanoparticles 22a therein, the substrate 24 is
preferably a solid substrate, wherein at least the surface in which the
metallic nanoparticles 22a are arranged is made of an insulator. Since
this substrate is used as the photodetector, the substrate is preferably
made from a material that does not absorb light (e.g., light in the
visible region to the near infrared region) incident from the outside,
for example comprising a transparent substrate. Since the nano-chain unit
22 is manufactured using the semiconductor fine processing technique
(e.g., electron beam lithography, sputtering, or the like) as shown in
FIG. 16, the substrate needs to withstand these processings. Accordingly,
for example, a glass substrate, a quartz substrate, a sapphire substrate,
or the like is used as the substrate 24.

[0101]For the manufacturing method when forming the nano-chain unit 22 on
the substrate 24, the nano-chain unit 22 is fabricated using the
already-described manufacturing method of FIG. 16.

[0102]A plurality of bottlenecks in the linked metallic nanoparticles are
preferably arranged on a straight line as described above, however, as
shown in FIG. 3A, the overall length X of the nano-chain unit 22 on this
straight line is set in accordance with a resonance wavelength. As
apparent from FIG. 17, FIG. 18, and the like, the length X of the
nano-chain unit 22 is adjusted in accordance with the size of a metallic
nanoparticle to be linked, the number of metallic nanoparticles to be
linked, or the like. If the length of the nano-chain unit 22 is
lengthened, the plasmon resonance absorption wavelength of the metallic
structure will shift to the long wavelength side. Since this point has
been described in conjunction with FIG. 17, FIG. 18, the another
description thereof is omitted here.

[0103]The wavelength of light to be absorbed can be varied by varying the
length X of the nano-chain unit 22, and the application of this knowledge
allows for detection of light.

[0104]On the other hand, the current detection probes 21, 23 are formed
from a metallic pattern and comprise, for example, a metal multilayer
which is formed by forming Cr (chromium) with the film thickness of 5 nm
on the substrate 24 and then depositing Au (gold) with the film thickness
of 40 nm on the Cr. The metallic nanoparticle 22a is formed in a
polygonal shape, and is square in this embodiment. These current
detection probes 21, 23 are formed by sputtering or vapor deposition.

[0105]Moreover, each of the current detection probes 21, 23 has a corner
portion whose tip is formed with a predetermined angle, and this corner
portion is arranged so as to face a corner of the metallic nanoparticle
22a present at the tip of the nano-chain unit 22. In FIGS. 19A, 19B, the
current detection probe 21 constitutes a probe on the negative electrode
side, and the current detection probe 23 constitutes a probe on the
positive electrode side. However, instead of arranging one probe for the
positive electrode and one probe for the negative electrode as in FIGS.
19A, 19B, a plurality of probes may be arranged for each of the
positive/negative electrodes to detect the current.

[0106]Moreover, the gap width t between the corner portions of the current
detection probes 21, 23 and the corner of the metallic nanoparticles 22a
at the tip of the nano-chain unit 22 is formed in the range of
0<t≦10 (nm). This gap may be present at least between the
current detection probe 21 and the metallic nanoparticle at one end of
the nano-chain unit 22 or between the current detection probe 23 and the
metallic nanoparticle at the other end of the nano-chain unit 22. In
other words, the current detection probes 21, 23 and the nano-chain unit
22 just need not to be completely short-circuited to each other.
Accordingly, for example, if the gap width t between the current
detection probe 21 and the metallic nanoparticle at one end of the
nano-chain unit 22 is formed in the range of 0<t≦10 (nm), then
the gap width t between the current detection probe 23 and the metallic
nanoparticle at the other end of the nano-chain unit 22 is formed in the
range of 0≦t≦10 (nm). Here, a power supply 26 and a
resistor 25 are coupled in series between the current detection probes 21
and 23. A DC power supply is used as the power supply 26. While the
nano-chain unit 22 is not receiving light, the voltage V of the DC power
supply 26 is varied to detect the current I that flows between the
current detection probes 21, 23, and the current-voltage characteristic
(I-V characteristic) is measured. Then, the characteristic such as X1 of
FIG. 20 is obtained. Although schematically drawn here, the
current-voltage characteristic draws almost a staircase pattern.

[0107]Since the current detection probes 21, 23 and the nano-chain unit 22
have a gap therebetween and are not short-circuited to each other, a
current will not flow. However, a current will flow if the applied
voltage exceeds a certain voltage. On the other hand, if the nano-chain
unit 22 is irradiated with light, the plasmon resonance absorption at a
wavelength corresponding to the length X of the nano-chain unit 22 is
carried out and the polarization will occur. This will change the amount
of current flowing at a certain voltage. Therefore, by observing the
increase and decrease in the amount of current, an increase and decrease
in the light intensity of a wavelength at which the resonance absorption
is being carried out is detected.

[0108]FIG. 21, FIG. 22, and FIG. 23 show the results of this experiment.
In FIG. 21, a photodetector such as the one in FIGS. 19A, 19B was
fabricated, and the length X of the nano-chain unit 22 was adjusted to
set the plasmon resonance absorption near the wavelength of 1,000 nm.
Then, the current detection probe 23 and the tip on the right side of the
nano-chain unit 22 were short-circuited to each other, and the gap
between the nano-chain unit 22 and the current detection probe 21 was
irradiated with a laser beam having a wavelength of about 1,000 nm to
measure how the current-voltage characteristic fluctuates.

[0109]It is shown in FIG. 21 that, as the driving current for driving the
laser is varied, the initial voltage V0 , at which a current starts to
flow between the current detection probes 21, 23, will abruptly decrease
(shift to the left side in the view) when the laser driving current
becomes 59 mA or more. FIG. 22 is a graph showing the relationship
between the LD driving current and the initial voltage V0 of FIG. 21.
Note that the initial voltage V0 in the vertical axis of FIG. 22
represents the value obtained by reversing the positive/negative sides of
the voltage axis of FIG. 21, and here the upper in the vertical axis, the
smaller the value of the initial voltage V0 becomes.

[0110]The initial voltage V0 remains approximately 0 μV until the laser
driving current reaches the laser threshold current of 47.9 mA at which
the laser oscillation starts. However, the initial voltage V0 starts to
decrease around when the laser driving current exceeds the laser
threshold current of 47.9 mA, and the initial voltage V0 abruptly
fluctuates largely when the laser driving current reaches near 60 mA. In
this way, immediately after the oscillation of the laser beam starts, the
fluctuation in the initial voltage V0 also increases in accordance with
the intensity of the laser beam.

[0111]On the other hand, the photodetector of FIGS. 19A, 19B was
configured the same as in the above-described experiment with the laser
irradiation and then the current-voltage measurement was carried out
under irradiation of light from a light source lamp to obtain the result
of Y2 of FIG. 23, while the result of the current-voltage measurement
under no irradiation by turning off the light source lamp is Y1 of FIG.
23.

[0112]The curve Y2 under irradiation of light has a smaller initial
voltage, at which a current starts to flow, than Y1, and the graph as a
whole also has shifted to the left. Incidentally, when the circuit of
FIGS. 19A, 19B is short-circuited by short-circuiting the nano-chain unit
22 and the current detection probe 21 and short-circuiting the nano-chain
unit 22 and the current detection probe 23, the current-voltage
characteristic under irradiation of the light of the light source lamp is
represented by Z2, while the current-voltage characteristic under no
irradiation of the light of the light source lamp is represented by Z1.
In this way, when the circuit is short-circuited, there is little change
in the current-voltage characteristic regardless of being irradiated with
light or not. As apparent from the above, light irradiation causes a
potential difference in the gap.

[0113]FIG. 24A to FIG. 24c show modifications of the arrangement of the
current detection probes 21, 23 on the positive electrode and negative
electrode sides and the nano-chain unit 22. In FIG. 24A, the tip corner
of the metallic nanoparticle 22a at the end of the nano-chain unit 22 is
arranged so as to be sandwiched by the current detection probes 21, 23.
The gap width t between the current detection probes 21, 23 is formed in
the range of 0<t≦10 (nm) as described above.

[0114]In FIG. 24B, the current detection probe 21 is arranged facing one
(one side surface) of two opposing comers of the metallic nanoparticle
22a at the end of the nano-chain unit 22, while the current detection
probe 23 is arranged facing the other corner. In this case, at least one
of the gap width t between the metallic nanoparticle 22a and the current
detection probe 21 and the gap width t between the metallic nanoparticle
22a and the current detection probe 23 is formed in the range of
0<t≦10 (nm), and the other gap width t is formed in the range
of 0<t≦10 (nm). In FIG. 24c, the arrangement of the current
detection probes 21, 23 in FIG. 24B has been moved to the corner of the
metallic nanoparticle 22a in the center part of the nano-chain unit 22.

[0115]Incidentally, in the configuration of FIGS. 19A, 19B, one set of
photodetection unit including the current detection probe 21, the
nano-chain unit 22, and the current detection probe 23 is arranged on the
substrate 24. However, a photodetector capable of detecting various
wavelengths at one time can be configured by arranging a plurality of
nano-chain units 22 on a substrate, in which each of the nano-chain units
22 has its overall length varied, and a pair of positive/negative current
detection probes arranged thereto.

[0116]Although the planar shape of the metallic nanoparticle in the
nano-chain unit 22 is square in the above-described embodiments, the
metallic nanoparticle can be formed in other shape. As described in the
metallic structure of the present invention, the metallic nanoparticles
of FIG. 1A to FIG. 1C may be used. The planar shape of each of the
metallic nanoparticles is formed in a circular shape in FIG. 1A, in an
isosceles triangle shape in FIG. 1B, and in a rhomboid shape in FIG. 1C.
When these shapes are used, the nano-chain unit will have the operational
effects described in the above metallic structures.

[0117]In this way, the present invention includes various kinds of
embodiments that have not been described here, of course. Accordingly,
the technical scope of the present invention shall be defined only by the
appended claims that are appropriate from the above description.